(1) Field of the Invention
The present invention relates to an imaging device formed by one-dimensionally or two-dimensionally arranging on a semiconductor substrate a plurality of unit cells that subject incident light to photoelectric conversion. In particular, the present invention relates to a technique for preventing loss of shadow detail in an image underexposed due to strong incident light.
(2) Related Art
In recent years, such imaging devices as home video cameras and digital still cameras have been popularly widespread.
Theses imaging devices may include an image sensor featuring an amplifier.
Although having advantageous characteristics such as low noise, an image sensor featuring an amplifier suffers from the problem that underexposure occurs due to strong incident light, resulting in loss of shadow detail of an image.
Japanese Laid-Open Patent Application No. 2000-287131 describes an overview of a CMOS image sensor that is an image sensor featuring an amplifier, discusses the problem mentioned above, and discloses a CMOS image sensor that can solve the above problem by detecting, for each pixel sensor, strong incident light based on an output voltage at reset, and replacing the output voltage at reset with another voltage.
According to Japanese Laid-Open Patent Application No. 2000-287131, an output voltage at reset is used to detect a pixel sensor for which shadow detail loss of an image may occur. A change in the output voltage at reset however is a direct cause for shadow detail loss of an image. Even a subtle change in the output voltage at reset directly affects brightness information.
Such a change in an output voltage at reset can be detected only when the change is large enough to be detected. It is therefore impossible to completely eliminate an adverse effect by such a change in an output voltage at reset.
Further, an output voltage at reset changes acutely when strong light is incident. Therefore, accurate detection of such a change is not easy, and thus effectively preventing shadow detail loss of an image is not an easy task.
Assume for example that a photograph of a subject composed of a sufficiently bright middle portion and surrounding peripheral portions that get gradually darker as closer to the edge of the subject is taken by the CMOS image sensor disclosed in Japanese Laid-Open Patent Application No. 2000-287131. In this case, the image sensor can prevent underexposure in the sufficiently bright middle portion, but fails to prevent underexposure in a peripheral portion surrounding the middle portion, so that the peripheral portion surrounding the middle portion appears to be darker than a further peripheral portion closer to the edge. Depending on the degree of underexposure occurring in the peripheral portion appearing darker, this portion may be viewed as a ring-shaped black area in which shadow detail is lost.